In the current information technology era, plagiarism is a significant and critical issue in research. Plagiarism detection tools are essential in identifying instances of plagiarism. This study compared the similarity index generated by three leading plagiarism detection software platforms: iThenticate, Ouriginal, and Turnitin. Ten original documents (N = 10) were selected for analysis across the three software programs. The process involved first analyzing all documents with Ouriginal, then checking the same documents, followed by iThenticate, and Turnitin. These software programs generated originality reports detailing the number of matching sources, similar word counts, and an overall similarity index as a percentage. To detect notable differences within the dataset, a one-way ANOVA and a Tukey (HSD) post-hoc analysis were conducted. The threshold for statistical significance was established at p<0.05. Statistical analysis revealed that while there was a significant variance in the similarity index across the tools iThenticate, Ouriginal, and Turnitin (F (2, 27) = 5.436, p = .010), there were no notable differences in the sources they matched (F (2, 27) = 1.289, p = .292). This suggests that the plagiarism detection capabilities may vary significantly among these tools, but the sources they identify as matches are largely consistent. However, the average values indicated that Turnitin had the highest mean similarity detection followed by iThenticate, and then Ouriginal. In this study, evaluating the similarity index can help verify the effectiveness of anti-plagiarism tools and safeguard researchers against committing plagiarism. [ABSTRACT FROM AUTHOR]